When a recording film constituted by a thin film of a phase-change recording material or the like formed on a substrate is irradiated with a laser beam and subjected to localized heating, the recording film can be made to change to states with different optical constants due to differences in the irradiation conditions. Thus, the optical information recording medium (hereinafter also written as the optical recording medium) is subjected to the optical recording, erasure, rewriting, or reproduction of information by using a laser beam. Optical recording media have been widely researched and developed, and BDs (Blu-ray discs) and DVDs and CDs and the like have been commercialized.
With phase-change optical recording media, information is recorded as a result of a phase-change material, which constitutes a recording film, being made to undergo a change in state between a crystalline phase and a noncrystalline phase, for example, with the heat generated from laser beam irradiation. Further, reproduction of information is performed by detecting a difference in reflectance between the crystalline phase and the noncrystalline phase.
Among optical recording media, a rewritable optical recording medium is capable of information erasure or rewriting as a result of using, as the recording film, a phase-change recording material in which an reversible phase change is generated. In the case of a rewritable optical recording medium, the initial state of the recording film is typically the crystalline phase. If information is recorded, a high-power laser beam is irradiated to melt the recording film and the laser irradiated portion is then shifted to the noncrystalline phase by means of rapid cooling. If, on the other hand, information is erased, a laser beam of a low power in comparison with the laser beam used during recording is irradiated to raise the temperature of the recording film, and the laser irradiated portion is subsequently shifted to the crystalline phase through gradual cooling. Further, by irradiating a laser beam which is power-modulated between a high power and a low power on a recording film, the recording of new information while erasing recorded information, that is, rewriting, can be performed. In the case of a rewritable optical recording medium, the noncrystalline portions are marks and the crystalline portions are spaces.
Further, in the case of a recordable optical recording medium for which a material which generates an reversible phase change is used as the recording film, rewriting of information is not possible and information can be recorded only once.
For both a rewritable optical recording medium and also a recordable optical recording medium, a metal film of high thermal conductivity is typically used in addition to a recording film with the object of performing efficient cooling of the heat during recording.
The reproduction of information recorded on an optical recording medium is performed by checking a difference in reflectance between the crystalline layer and the noncrystalline layer. More specifically, the reproduction of information is performed by performing detection by taking the strength of reflected light from the optical recording medium as a signal when a laser beam, set with a certain fixed reproduction power, is irradiated on the optical recording medium.
As technologies for increasing the capacity of an optical recording medium, various technologies have been investigated. For example, there exists a method for reducing the minimum size of the mark length and space length to raise the recording density. This method is confronted not only by the problem that the S/N ratio of the reproduction signal drops, but is also subject to a phenomenon whereby heat, which is generated when marks are recorded, is diffused by the space parts and the process of cooling fore-and-aft adjacent marks is affected, that is, thermal interference is generated. When thermal interference is generated, the edge positions of the marks shift and the error rate during reproduction rises, which is problematic.
Therefore, a method has been disclosed according to which, if marks and spaces are formed by driving the laser power at two values, namely, a high power and a low power, recording is performed by changing the positions of the leading end parts of the marks according to the mark length of the recorded marks and the space length of the spaces directly in front of the marks, and changing the positions of the trailing end parts of the marks according to the mark length of the recorded marks and the space length of the spaces directly behind the marks (see Patent Literature 1, for example). Thus, by modulating the control parameters of the recording pulse which is selected when marks are recorded, variations in the edge positions of the marks caused by thermal interference between the marks during high density recording are compensated for.
Further, even if marks and spaces of the correct lengths are formed, due to the frequency characteristics of the reproduction optical system which are determined by the size of the optical beam spot, there is the problem that the edge positions of the short marks and spaces detected during reproduction are reproduced with discrepancy from the ideal values. This disparity between the detected edge and the ideal value is generally referred to as intercode interference. When the marks and spaces are made small in comparison with the optical beam spot, the intercode interference is substantial, and there is a problem in that jitter during reproduction increases and the error rate rises. Hence, a method has been proposed to determine the most probable signal series from a reproduction signal waveform obtained from the optical recording medium by means of a signal processing system called PRML (Partial Response Maximum Likelihood) which is one example of a maximum-likelihood decoding method.
For example, in an optical system which uses a laser beam with a wavelength of 405 nm and an objective lens with an NA (numerical aperture) of 0.85, it is preferable to adopt a PR (1, 2, 2, 1) ML system in order to record information at a capacity of 25 GB per side of a BD with a diameter of 12 cm and to accurately reproduce recorded information. Furthermore, in order to record information at a capacity of 33.4 GB per side using the same optical system, it is necessary to shorten the mark length and increase the line density. In this case, a PR (1, 2, 2, 1) ML system is desirably adopted for the processing of the reproduction signal.
Furthermore, Patent Literature 2 discloses a recording control method with which there is no reproduction signal jitter and which uses a PRML system to optimize the control parameters of the recording pulse when recording information. With the recording control method of Patent Literature 2, the signal waveform is estimated from the reproduction signal waveform by means of the PRML system and the control parameters of the recording pulse are optimized to minimize the error reproduction probability.
As another measure for increasing the capacity of an optical recording medium, there exists a method which records or reproduces information on a plurality of information layers by means of a laser beam which enters from one side of a rewritable optical recording medium which comprises a plurality of information layers.
For example, if an optical recording medium comprises two information layers, the recording capacity doubles. In an optical recording medium which records or reproduces information on two information layers by means of a laser beam which enters from one side of the optical recording medium, the recording or reproduction of information on the information layer farthest from the entry side (hereinafter the first information layer) is performed by means of a laser beam which passes through the information layer closest to the entry side (hereinafter the second information layer). That is, when the transmittance of the second information layer is low, because the energy of the laser beam which reaches the first information layer is attenuated, the reflectance from the first information layer is substantially small and the signal quality of the information during reproduction suffers. Note that in the following description, reflectance refers to a substantial reflectance which includes attenuation caused by passage through another information layers. Further, reflectance which does not include attenuation caused by passage through another information layer is called film reflectance.
When the transmittance of the second information layer is low, there is an increased amount of laser power required to suitably record information on the first information layer. If the laser power exceeds the limits of the recording device, suitable recording cannot be achieved and the quality of the information is adversely affected at the time of recording. Hence, the second information layer preferably possesses a transmittance which is as high as possible.
In order for the information layer on the entry side of the laser beam to have a high transmittance, in the information layer on the entry side of the laser beam, the extinction coefficient is preferably large and the thickness of the metal film is preferably small. However, in a recordable optical recording medium, when the metal film is low in thickness, the rate of cooling of the heat generated during recording is slow. For this reason, the transmission of heat to outside the areas where the laser beam is irradiated is large and the reproduction signal worsens as a result of blurring of the boundaries between the marks and spaces. Therefore, usage of a recording pulse to achieve a rapid cooling temperature change from the time information is recorded on the information layer farthest from the entry side when information is recorded on the information layer closest to the laser beam entry side has been proposed (Patent Literature 3).
Patent Literature 4 discloses recording information, relating to control parameters of a recording pulse for suitable recording of an optical recording medium, in information units within a predetermined area of the optical recording medium.
In addition, Patent Literature 5 discloses, in an optical recording medium comprising a plurality of information layers, calculating, for each information layer, parameters of each power of a recording pulse which is modulated with a plurality of powers with different levels, and recording information relating to the ratio between a specific power of each information layer and the highest level power in information units within a predetermined area of the optical recording medium.
However, with the technology of Patent Literature 3, if information is recorded on an optical recording medium comprising three or more information layers in order to further increase the capacity, there is a problem in that the erasure performance suffers. In other words, in order to further increase the transmittance, a recording film made from a phase-change material with a high extinction coefficient must have a small thickness like a metal film. However, generally, in a rewritable optical recording medium, when a recording film made from a phase-change material has a small thickness, the crystallization rate is low. A phase change from the noncrystalline phase to the crystalline phase is hard to produce and the information erasure performance is compromised.
In the second information layer of an optical recording medium comprising two information layers, the effect of the erasure performance being compromised is not problematic. However, in order to realize an optical recording medium which comprises three information layers, the transmittance of the information layer closest to the laser beam entry side (hereinafter the third information layer) must be higher than the transmittance of the second information layer. Therefore, the thickness of the recording film of the third information layer is thinner than the thickness of the recording film of the second information layer and it is hard for the erasure performance of the third information layer to satisfy the level that is actually required.
In addition, in an optical recording medium which comprises three information layers, the reproduction of a signal which is recorded on the second information layer is performed by means of a laser beam whose light quantity is attenuated by passing through the third information layer. Hence, the substantial reflectance of the second information layer is reduced. Typically, the ratio between the reflectances of two different information layers is preferably between 0.5 and 2.0. To this end, the film reflectance of the second information layer must be higher than the film reflectance of the third information layer. However, in an information layer for which light transmittance is required, when the film reflectance increases, there tends to be a reduction in the ratio between the reflectance of the recording film which is the crystalline phase and the reflectance of the recording film which is the noncrystalline phase. Hence, the signal amplitude is reduced and there is a problem in that the reproduction signal quality of the second information layer suffers.